1. Field of the invention
The invention relates to a high pressure mercury ultraviolet lamp which is used for a light source of a UV curing device and the like.
2. Description of Related Art
In a conventional high pressure mercury ultraviolet lamp, to facilitate discharge, on the tips of the upholding parts of the electrodes there are usually double or triple coils to which emitter material of barium oxide or the like is applied. However, this emitter material heats up during operation of the lamp within the discharge vessel and sprays onto the walls of the discharge vessel where it adheres, causing the UV radiation transmission factor of the wall of this discharge vessel to be reduced. As a result, the light output necessary for treatments using UV radiation is reduced and the service life of the lamp is shortened.
Recently, together with the advancing reduction in the size of devices which have UV lamps, the lamps continue to shrink. In high pressure mercury ultraviolet lamps of the short arc type with an inside volume of greater than or equal to roughly 2.5 cm.sup.3 the above described arrangement in which the emitter material is applied to the electrodes is used in practice. However, in high pressure mercury ultraviolet lamps with smaller dimensions, the application of the emitter material to the upholding parts of the electrodes entails difficulties with respect to the arrangement. For smaller high pressure mercury ultraviolet lamps, due to the smaller area of the inside wall of the discharge vessel, even if the emitter material can be applied to the upholding parts of the electrodes, the above described spraying of emitter material onto the wall surface has a great effect on the reduction of the amount of light.
The emitter material which is deposited on the wall of the discharge vessel, especially in the case of using UV radiation with wavelengths of less than or equal to 400 nm, has a higher absorption factor for light in the UV range with short wavelengths than for light in the range of visible radiation. The effect of deposition of the layer onto the wall of the discharge vessel by the spraying of emitter material is therefore very great. Furthermore, in the case of an inside volume of the discharge vessel of less than or equal to 2.5 cm.sup.3 it is difficult to obtain dimensional accuracy.